Gallium arsenide, along with derivatives such as gallium aluminum arsenide, is used in a wide range of electronic and optical applications. During the manufacture of gallium arsenide devices, as much as 90% of the gallium arsenide becomes waste. Gallium is not readily available from primary sources, but is produced in relatively small quantities as a byproduct from aluminum, zinc and phosphorus operations. Since gallium is expensive and relatively scarce, gallium arsenide waste represents a valuable source for gallium.
Gallium arsenide is a very stable material and methods for its recovery often involve very stringent means such as vacuum thermal decomposition or treatment with very reactive agents. Thus Abrjutin et al, U.S. Pat. No. 4,362,560, discloses a vacuum-thermal decomposition process for treating various high grade gallium arsenide wastes, and also references various prior art processes, including oxidation of gallium arsenide under anhydrous conditions with chlorine. Abrjutin et al further describe preliminary hydrochemical treatment to remove impurities, involving treatment with an aqueous solution of hydrochloric acid in the presence of an oxidizing agent. Nitric acid or hydrogen peroxide are preferred for use as the oxidizing agent. The plates of gallium arsenide after this hydrothermal treatment are subjected to vacuum-thermal decomposition. Bird et al, Production of High Purity Gallium from Scrap Gallium, SME Minisymposium on "The Hydrometallurgy of The Rarer Metals", Dallas, 1982, pp. 59-64, describes various sources of gallium arsenide scrap, and a process to produce high purity gallium therefrom; the process includes disassociation by leaching in hot aqua regia (4HCl:1 HNO.sub.3), and neutralization of the acid solution with NaOH to precipitate Ga(OH).sub.3. The byproduct salt solutions, the acid fumes and NO.sub.x emissions make such systems difficult to deal with from both a health and environmental point of view. The dissolved arsenic and other metal ions (from dopants and co-metals, etc.) cause serious problems in disposal of the reaction mixture. The product solution, which may contain only a percent or so of gallium, must have all of its acid neutralized to recover gallium hydroxide as gallium hydroxide solid. Also the solid is gelatinous and extremely difficult to filter.
There is extensive literature on chemical etching of various semiconductor materials, such as Werner Kern, Chemical Etching of Silicon, Germanium, Gallium Arsenide, and Gallium Phosphide, RCA Review, Vol. 39, June, 1978, pp. 268-309, which in discussing general etching mechanisms of semiconductors, indicates that they typically involve oxidation-reduction reactions, followed by dissolution of the oxidation products, frequently by complexing. In the case of silicon and germanium the oxidation agent is frequently HNO.sub.3, and the complexant is HF. NaOH-H.sub.2 O.sub.2 and H.sub.2 SO.sub.4 -H.sub.2 O.sub.2 -H.sub.2 O solutions are listed among the most commonly employed etchants for GaAs. The Electrochemistry of Semiconductors, Ed. by P. J. Holmes, Academic Press, London and New York (1962), at pages 367-375, has a section on Etchants for the More Important Semiconductors, and at page 372 lists an NaOH and H.sub.2 O.sub.2 solution and an HCl, HNO.sub.3 and H.sub.2 O solution among those for gallium arsenide. A recipe for chemical polishing Indium Telluride includes Br.sub.2 and acetic acid saturated with citric acid. A citric acid-hydrogen peroxide-water system for preferential etching of GaAs is described by Otsubo et al, J. Electrochem. Soc., 125 (5), pp. 676-680.
Some hydroxamic acids have been utilized in the art for extraction purposes. U.S. Pat. No. 3,821,351 issued June 28, 1974 to M. F. Lucid discloses certain N-substituted hydroxamic acids useful as extractants for the recovery of copper, molybdenum, uranium, iron and vanadium. U.S. Pat. No. 3,971,843 issued July 27, 1976 to J. Helgorsky et al discloses a solvent extraction process employing certain substituted hydroxyquinolines for the recovery of gallium from aqueous alkaline solutions.
Xiang et al in Acta Metallurgica Sinica 18 (2), 221, (1982) describe the use of a certain undefined fatty hydroxamic acid for the recovery of gallium from aqueous acid solutions.
Iwaya, Japanese Patent No. SHO60(1985) 245736, Appl. No. Sho. 59(1984)-101504, published Dec. 5, 1985, discloses a method of recovering gallium, using hydroxamic acids, described as having --C(O)NHOH groups, from high-basicity aqueous sodium aluminate solutions.
We have now discovered a process which can be adapted so that GaAs is easily dissolved using mildly reactive reagents, and the gallium and arsenic are separated without the need for use of large amounts of neutralization reagents, and with minimal (theoretically zero) emissions of hazardous chemicals into the environment. The process can avoid harmful NO.sub.x emissions and provide for regeneration of the dissolution reagent for re-use.